Digital video coding is an active field of research and standardization, since digital video is used in many consumer appliances such as laptop or desktop computers, digital cameras, digital recording devices, tablet computers.
In order to efficiently store and transmit digital video data, the digital video data is compressed.
Various compression standards have been developed, and the most recent one, the High Efficiency Video Coding (HEVC), which is presently under development, reaches higher compression efficiency than previous standards, and can be applied for very large picture resolutions.
Similarly to other compression methods, HEVC uses spatial and temporal prediction to reduce the redundancy of the video data and to achieve compression. The spatial prediction consists in predicting picture portions from other portions of the same picture, at the same temporal instant of the video. The temporal prediction consists in predicting a portion of a picture of the video from a portion of another picture of the video sequence, pointed at by a motion vector. The temporal prediction is also called inter prediction.
HEVC has an aim of further improving compression that is achieved by using hierarchical coding units, the basic unit used for carrying information related to prediction processes, called prediction unit, being of variable size and shape (either rectangular or square, with a variable size as from 4×4 to 64×64 pixels), so as to better take into account the content of a video sequence.
Moreover, HEVC proposes mechanisms to further increase the compression by reducing the bitrate of motion information to be transmitted. To this end, in the inter prediction modes, in particular the modes called “merge” and “skip”, the motion information associated to a prediction unit is predicted from a candidate list. The candidate list includes motion information of already processed neighbouring prediction units of a prediction unit to be encoded, either in a spatial neighbourhood of the prediction unit to be encoded or in a temporal neighbourhood of the prediction unit to be encoded. The candidate list being predetermined, only an index indicating the position of the best suited motion information predictor for the motion information of the current prediction unit to be encoded is actually encoded and transmitted to the decoder. The encoding chosen is a specific entropic encoding called context-adaptive binary arithmetic coding (CABAC) which adapts a probability model used by an arithmetic encoder according to a local context. The entropic encoding using CABAC, as applied in HEVC, brings a particular advantage in terms of compression enhancement if the candidate list is ordered according to the relevancy of the candidate motion information predictors, the first candidate in the list being the most relevant.
The standard HEVC is dedicated to mono-view video compression, since it is adapted to compress a video stream comprising a single video sequence. An extension of HEVC standard, currently under development, is directed to the compression of multi-view video streams, which comprise N video sequences, with N at least equal to two, captured simultaneously, typically by N cameras which are positioned at different spatial positions. The views are rectified, therefore the cameras are assumed to be aligned. A block of a picture in a first view corresponds to a block displaced on the horizontal axis of a second view, the displacement depending on the distance between the cameras. Several applications can be envisaged for the multi-view video coding, also called 3D video coding, such as stereoscopic television and multi-view 3D television.
In order to take advantage of the potential redundancy offered by the presence of multiple views, a disparity compensated prediction mode has been added to the HEVC inter prediction modes. The disparity compensation refers to motion compensation between prediction units of sequences corresponding to different views: a prediction unit of a current view has a disparity motion vector pointing to a portion of a reference picture of a different view. Further, the list of candidates for motion information prediction comprises, for a current prediction unit to be encoded, the motion information associated to the prediction unit of another view corresponding to the current prediction unit.
There is still a need to improve compression, both in mono-view video coding and in multi-view video coding.